Cell balancing is the BMS function that evens out the state of charge of every cell in a lithium battery. It improves battery life because it lets you use the battery's full capacity, stops some cells from working harder than others and slows down wear over the cycles. It's probably the function that does most to decide how many years a battery lasts.
This article covers why cells drift out of balance, how balancing extends their life, the difference between passive and active balancing, and how we handle it at NCPower.
Cell balancing in one sentence
It's keeping every cell in the battery moving at the same pace, so none of them holds the others back.
The problem: why cells drift out of balance
An industrial lithium battery is made of many cells connected in series. Even when they come from the same factory, they're never identical: they have small differences in capacity, internal resistance and self-discharge. With use and charging cycles, those differences leave some cells more charged than others.
Picture the battery as a team of runners tied to the same rope. If one tires first (has less charge), the whole team has to stop, even though the others still have energy left. A battery works the same way: discharge stops when the weakest cell runs empty, and the energy left in the others can't be used.
How balancing improves battery life
- It uses the full capacity. With the cells evened out, the battery doesn't stop early because of a single cell.
- It protects every cell. It stops some cells from being overcharged or discharged more than others, which is what damages them for good.
- It improves safety. With no cell pushed beyond its limits, the risk of overheating from overcharge or deep discharge goes down.
- It slows degradation. The cells age at the same pace, so the battery keeps its capacity for more cycles.
The two classic methods: passive and active balancing
1. Passive balancing: the basic standard
It's the most common method. When one cell reaches its maximum voltage before the others, the BMS burns off its excess energy in a resistor, as heat. That heat stays on the BMS board itself and warms up the rest of the electronics.
- Advantage: it's simple and cheap.
- Disadvantage: it's slow. To avoid burning the components, it usually works with very low currents and only at the end of charging, so it corrects imbalances slowly and makes charging take longer.
2. Active balancing: the ideal solution?
Instead of throwing away the surplus energy, it moves it from the more charged cells to the less charged ones.
- Advantage: in theory, it doesn't waste energy.
- Disadvantage: in large industrial batteries it falls short. The transfer channels usually move very small currents, so it's too slow to correct large imbalances. It also adds electronics and complexity, and with them more points of failure.
| Passive balancing | Active balancing | NCPower balancing | |
|---|---|---|---|
| How it works | Burns off excess energy as heat | Moves energy between cells | Dissipative, with the heat kept away from the electronics |
| When it works | Usually only at the end of charging | During charging | During charging and discharging |
| Balancing current | Very low | Low | High: 1 A per cell |
| Main advantage | Simple and cheap | Doesn't waste energy | Corrects fast without heating the BMS |
The NCPower approach: dynamic, powerful, decoupled balancing
After years of experience in professional electric mobility, at NCPower we decided not to settle for the standard. Our system combines the reliability of the dissipative method with much more power and better thermal management.
Heat kept away from the electronics. The big flaw of passive BMSs is that their resistors heat up the main board. We move the balancers to an external module, protected by an aluminium casing and thermally isolated from the BMS. That lets us balance with much higher currents without shortening the life of the electronics.
Balancing while charging and discharging. Most BMSs wait until the battery is almost full before they start balancing, which makes charging take longer. Ours is dynamic:
- It balances while charging, from the very start, so the machine spends less time plugged in.
- It balances while discharging, while the machine is working, evening out the differences between cells on the go.
The result, in numbers. With a multi-channel architecture and 1 A per cell, in a 300 Ah pack we correct a 1% imbalance in about 3 days, compared with 30 to 60 days for conventional passive balancing. Balancing during both charging and discharging cuts charging time by up to 40%, and good balancing extends battery life by around 30% in cycles.
By balancing all the time, the battery doesn't reach the end of the day with critical imbalances: fewer unexpected stops and 100% of its capacity available for much longer.
What to ask about balancing before you buy
- When does the BMS balance? Only at the end of charging, or during use as well?
- What balancing current does it use? It decides how long it takes to correct an imbalance in a large battery.
- Where does the balancing heat go? If it stays on the BMS board, it limits the current and the life of the electronics.
- Can I see the status of each cell? With remote monitoring you spot an imbalance before it stops the machine.
For an overview of everything a BMS does (monitoring, protection, balancing and communication), see our guide what is a BMS and why it decides how long your lithium battery lasts. And to see the full architecture of our system, visit our technology page.
Where do we start?
Replacing a lead-acid battery? Send us a photo of your current battery's nameplate and we'll tell you which lithium battery matches it.
Designing a new vehicle or machine? Tell us its discharge peaks, its charging window and how it communicates, and our engineers will explain how to integrate the battery and its BMS. You can also ask us for a preliminary consultation.
You can contact us here.